1. Field of the Invention
The present invention relates to the disposal, by injection, of organic biowaste (such as raw municipal sewage, organic farm waste, and the like) into a suitable underground formation where the biowaste may degrade into methane and other gases. More particularly, the present invention involves the use of underbalanced drilling techniques and technology to drill the reservoir zone that is to be injected with the biowaste. The invention also includes pre-treatment of the biowaste material to insure that its particle size is smaller than the pore throat size and/or fracture aperture of the underground formation where it will be injected.
2. Prior Art
The prior art discloses methods of treatment of bio-waste wherein the biowaste is injected underground at a pressure sufficient to induce fracturing in the underground rock formation. The biowaste is injected into the rock fractures where any organic material degrades and any inorganic material is trapped within the rock matrix. Alternate methods described within the prior art include the following.
Hamilton U.S. Pat. No. 3,724,542 discloses a process of injecting biowaste in the form of “activated sludge” into existing wells in depleted hydrocarbon-bearing formations, such as oil shales or exhausted petroleum fields. This process requires the high-pressure fracturing of the shale in order to induce permeability for the sludge injection.
Bilak et al U.S. Pat. No. 6,002,063 discloses a method for creating a highly viscous slurry of biowaste or other disposables, then injecting the slurry into deeply buried strata, with the injection pressure being greater than or equal to the fracture or overburden pressure, i.e., far greater than the natural water pressure of the target strata.
Bruno et al U.S. Pat. No. 6,491,616 B2 discloses a method for creating a slurry of biosolids suitable for injecting, selecting an injection formation below a ground surface (preferably a natural gas formation in a gas accumulation zone), and injecting the biosolids slurry into the injection formation at a pressure sufficient to create and maintain fractures within the selected injection formation, to allow degradation of the injected biosolids slurry.
Chesner U.S. Pat. No. 5,139,365 discloses a process for injecting selected wastes (including organic biowaste) into the inherent void space found within municipal solid waste landfills. The process involves injecting the waste under high pressure to either permeate or compact the existing landfill components.
Alexander et al U.S. Pat. No. 5,734,988 discloses a method for disposal of oil field waste or any waste slurry stream by injection into underground formations that are preferably underpressured, highly permeable, highly porous, dipping in angle, and highly fractured. The injection slurry itself is overpressured compared to the formation, with the hydrostatic pressure head of the slurry column sufficient to induce injection.
The primary problem with these and other methods of drilling and biowaste injection is that overbalanced drilling procedures damage the structure of the receiving formation and reduces the formation's permeability during the drilling and completion process. For example, skin damage of the reservoir can occur when drilling overbalanced in a permeable reservoir, when the drilling mud which consists of very fine particles is pumped downhole to transport rock cuttings out of the hole being drilled actually filters into the formation as a filtrate (liquid) which leaves behind a filter cake, composed of the solids that were in the mud. (Skin damage is a measure of the reduction of a formation's permeability that is caused by drilling.) This filter cake plugs the formation and would inhibit future injectability of a biowaste slurry.
There are expensive and complicated completion methods generally understood and available to mitigate some of the challenges of drilling overbalanced, as listed above, but there is still much room for improvement in the field of drilling and injection of biowaste. The problem does not lie with the decomposition of biowaste into methane and other gasses, but rather with how the formation that is to receive the biowaste is drilled and how the biowaste is prepared.